The present invention relates in general to electro-optical document reading systems such as those used in facsimile equipment, optical character recognition, and image digitization. More particularly, the present invention relates to scanning or electronicly reading documents by utilizing a linear photosensor array and physically transporting a document past an optical system to achieve two dimensional viewing (or scanning). Still more particularly, the present invention relates to a method of operating a document feed subsystem which is part of an optical imaging system in order to achieve variable resolutions of feedwise scanning.
In many electro-optical document reading systems, light is directed onto a scanwise line of a document, and light is reflected from each elemental area of the line of the document in accordance with the color or blackness of the elemental area. The reflected light is fed through an optical system, usually including a spherical lens, to an electrical apparatus in which the reflected light is converted to electrical signals. In facsimile equipment these electrical signals are used to reproduce the document at a remote location. Typical of such an electrical apparatus is a photo-electric device, positioned behind the lens, which converts light reflected from each unit area of a scanwise line on the document to electrical signals. One suitable photo-electric device comprises an integrated circuit chip having a large number of tiny photosensitive elements arranged in a line, each element receiving light from a unit area of each scanwise line. In the discussion to follow, the direction perpendicular to the photo-electric device's linear axis will be termed "feedwise". The present invention relates to the control of scanning resolution in the "feedwise" (or "lengthwise") direction.
Systems characterized by the latter mentioned characteristics are well known in the prior art. For example, a facsimile system having the characteristics previously described is taught in U.S. Pat. No. 4,220,978, issued Sept. 2, 1980 for an Electro-Optical Document Reader.
In the field of facsimile telecommunications, most presently offered equipment is compatible with one of the international facsimile transmission procedures promulgated by the CCITT. The CCITT is the international body (Consultative Committee on International Telephone and Telegraph) that formulates compatibility standards particularly for communication between countries. The CCITT has developed standards for facsimile machines known as Group 1, Group 2 and Group 3, enabling machines of different manufacturers to communicate with each other. Group 1 machines conform to the first CCITT standard set in 1968. They communicate in six minutes per page and employ a technique called frequency modulation (FM). Group 2 machines conform to the CCITT Standard set in 1976 and are capable of three minutes per page transmission. These machines employ a technique called amplitude modulation (AM) which results in faster transfer of ihformation. Group 3 machines conform to the latest adopted CCITT Standard established in November, 1980. They employ digital techniques in which the information on the page is converted to digital data. This results in transmission speeds of less than one minute per page and improved image quality.
In the past, CCITT standards have specified different "scanwise" (or horizontal) and "feedwise" (or vertical) resolutions. Scanwise (or horizontal) resolutions have been defined in dots per line (or dots per inch (DPI)) while feedwise (or vertical) resolutions have been defined in lines per mm (or lines per inch (LPI)).
At the present time, a new standard (Group 4) is being considered for adoption by the CCITT. Allowable CCITT Group 4 resolutions are 200, 240, 300, and 400 DPI (dots per inch) in both the scanwise (or horizontal) and feedwise (or vertical) directions. Ideally, it is desirous to implement these multiple resolutions within one facsimile device at a minimum cost. In prior art facsimile systems, such as that previously described, the horizontal photo-electric device is in a fixed position and a mechanical means is utilized to feedwise (or vertically) transport the document past the photo-electric device. Typically, stepper motors with a precise number of magnetic detent positions per shaft revolution are used to feedwise transport the document in these devices. Counting drive pulses supplied to the motor gives an implied knowledge of the feedwise distance travelled by the document with the necessary precision. More expensive and complex position sensing and feedback controls are thereby avoided.
By selectively energizing stepper motor poles, rotor positioning in intermediate positions is possible. Half steps, using digital drivers to energize selected motor poles, can be achieved at a small sacrifice in torque. By utilizing half steps and full steps, those skilled in the art will appreciate that it is quite easy to achieve feedwise scan resolutions of 200 or 400 DPI. Thus, a system may readily be designed to feedwise transport the document by one two-hundredth of an inch per full motor step by suitable selection of roller diameters and drive ratios. By utilizing half step control of the stepper motor, feedwise document movement of one four-hundredth of an inch may be readily obtained. Of course, higher resolution optics and finer photo-electric devices should also be used to uniquely resolve the smaller picture elements (pixels) in the applications which include four hundred DPI feedwise resolution.
The intermediate resolutions of 240 and 300 DPI feedwise are not uniquely available from a 200/400 DPI drive. In the prior art, these intermediate resolutions were achieved by utilizing analog drivers. However, the circuit costs and power levels in systems utilizing such analog drivers rises rapidly. Another technique for achieving the intermediate resolutions of 240 and 300 DPI feedwise is to utilize a motor step causing the document to be transported by the lowest common denominator or one twelve-hundredth of an inch. Although a drive system could be designed to achieve a transport distance per step of one twelve-hundredth of an inch, stepper motor speed limitations would cause unacceptable through-put limitations.
It is a general object of the present invention to overcome the drawbacks of the prior art by providing a low cost system for feedwise transporting a document at one of the four Group 4 resolutions.
It is a further object of the present invention to use a stepper motor driven system to feedwise advance a document past a horizontal photo-electric device at feedwise resolutions corresponding to 200, 240, 300, or 400 DPI.
It is still another object of the present invention to provide a method of operating a stepper motor driven paper drive system which utilizes combinations of half motor steps and full motor steps in order to approximate any one of four feedwise scan rates including 200, 240, 300 and 400 DPI.
These and other objects and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiment when read in conjunction with the drawings.